I have minute data in UTC format from Xetra, so half of the year the first bar is at 07:00, the other half it begins at 08:00.

I want Zorro to cut out the first and last 5 minute candles of each day because the data here is unreliable due to the widening spread.

I can use StartMarket and EndMarket but I can't seem to adjust this for DST. I guess it's fixed once it's set so every day begins at 08:05 for example, despite setting StartMarket to 0705 when the timezone changes.

I tried working with AssetMarketStart/AssetMarketEnd but same problem there.

Is there a way to do this?

This is just a wild guess, but can't you adjust StartMarket and EndMarket by placing them into an if-statement? So something like:

Nope, as mentioned in my post, this seems to be fixed once set. All bars then start at the same time throughout the year, even when adjusting StartMarket by an hour when dst() signals a timezone change

Just because your data is in UTC does not mean AssetMarketZone has to be in UTC. Change it to one with DST and adjust AssetMarketStart/AssetMarketEnd likewise.

Edit: If you're only doing the one asset, change BarZone, StartMarket, and EndMarket only for an easier time. Finally, make sure you have the appropriate BR_LOCAL configuration.

Correct way was just to set BarZone to CET and then set StarMarket and EndMarket just once. Don't need to adjust it when Zorro already adjusts the times to CET and thus StartMarket stays consistent throughout the year.

Note that different timezones have different date periods for DST:
https://en.wikipedia.org/wiki/Daylight_saving_time_by_country

If I had to trade a global portfolio, I would manipulate timeframes instead of using the standard StartMarket / EndMarket variables.